# Tissue to Air Ratio

Tissue-Air Ratio (TAR) is a term used in radiotherapy treatment planning to help calculate absorbed dose to water in conditions other than those directly measured.

## Definition

The TAR at a point in a water phantom irradiated by a photon beam is taken to be the ratio of the total absorbed dose at that point to the absorbed dose at the same point in a minimal-scatter phantom with just-sufficient build-up.[1][2]

Tissue-air ratio is defined as the ratio of the dose to water at a given depth to the dose in air measured with a buildup cap:

[TEX] TAR=\frac{D(f,z)}{D(f,0)} [/TEX]

where D(f,z) is the dose at a given depth z and distance focus-detector f; and D(f,0) is the dose in air (z=0).

Measurements for each are taken using an ion chamber for identical source to detector distances and field sizes.[3]